According to WHO chief researcher Dr Christopher Murray, the key factors are the big infectious disease killers of HIV/Aids, malaria and tuberculosis as well as big child killers including pneumonia and diarrhoea.

He said the healthy life expectancy in countries such as Botswana, Namibia and Zimbabwe has been reduced by 14 years because of the impact of Aids.

Alan Lopex, co-ordinator of the WHO Epidemiology and Burden of Disease team, said: "Healthy life expectancy in some African countries is dropping back to levels we haven't seen in advanced countries since Medieval times."

Why Africa fares badly

Infectious diseases

Conflict

Poor health systems

Poverty

Conflict is another major problem.

In bottom ranked Sierra Leone, eight years of civil war have seen nearly half the country's 4.5 million population displaced. A further 500,000 people are believed to have fled to neighbouring countries.

And at least 50,000 people have died in the fighting while there are an estimated 100,000 mutilation victims.

Truer picture

The new way of calculating the number of years that a person can be expected to live in full health, known as the DALE (Disability-Adjusted Life Years) system, helps give a truer picture of the health of a country than simply studying death rates, says Who.

To calculate DALE, the years of ill-health are weighted according to severity and subtracted from the expected overall life expectancy to give the equivalent years of healthy life.

The WHO rankings show that years lost to disability are substantially higher in poorer countries because some limitations - blindness, paralysis and the debilitating effects of several tropical diseases such as malaria - strike children and young adults.

People in the healthiest regions lose 9% of their lives to disability, compared to 14% in the worst-off countries.

There are 32 countries where disability-adjusted life expectancy is estimated to be less than 40 years.